The semiconductor industry is facing the challenging task of finding a candidate to replace silicon oxide, which has been the CMOS gate dielectric of choice for more than 50 years. A material with a dielectric constant (k) higher than SiO2 will allow making the dielectric thicker by a factor of k/k(SiO2), hence lowering the gate current leakage levels, and this without reduction of the capacitance and thus performance. The k-value is, however, only one of a list of requirements that includes thermal and chemical stability, a wide bandgap, limited interfacial layer formation, a controllable etch behavior, correct and stable threshold voltage, and a good reliability (i.e. over a 10-year life time).
At present, the semiconductor industry's focus is on HfO2 and HfSiOx, but parallel to the main stream development of HfO2 and HfSiOx, material screening of alternate dielectrics continues. The interest in these alternative materials arises from the fact that several issues remain with the Hf-based dielectrics. Among the issues are, targeting the correct threshold voltage, the limited improvement in k-value for HfSiOx, and the necessity of nitridation to stabilize the material and prevent phase separation.
One class of materials being investigated and reported on are the rare earth based high-k materials such as rare earth scandate materials (REScO3). First results obtained by Pulsed Laser and E-Beam Deposition suggest that these materials have a k-value comparable to HfO2 in combination with a high thermal stability.
Following deposition and gate patterning, the high-k dielectric material must be removed from the source and drain regions of the transistor. This removal must be accomplished without the loss of any of the underlying silicon, as well as little or no isolation oxide (field oxide) loss. In U.S. Pat. No. 6,656,852 a method is disclosed for etching a high-k dielectric. The method involves removing an exposed portion of a high-k dielectric layer from a substrate by wet etching with a solution comprising water, a strong acid, an oxidizing agent, and a fluorine compound. The etching solution provides selectivity towards the high-k film against insulating materials and poly-silicon however this method is not applicable to the rare earth scandate materials (REScOx) since these oxides are etched too fast and due to isotropic nature of wet etching an unacceptable undercut is formed under the gate.
The problem is removal of the rare earth based high-k dielectric selectively over the Si substrate and without making an undercut under the gate. Dry removal is impossible for these rare earth compounds since these compounds (e.g. Dy and Sc) form no volatile compounds. A wet removal is possible, but due to isotropic nature of wet etch it etches the high-k dielectric (e.g. DyScO3) dielectric also laterally, creating an undercut at the gate edge beneath the gate.